The 2021 American Housing Survey (AHS) included a supplemental survey of rental units subsidized through rental assistance programs administered by the U.S. Department of Housing and Urban Development (HUD). In addition, the U.S. Census Bureau used addresses provided by HUD to identify other units in its regular AHS sample that are subsidized by HUD. The 2021 AHS regular sample contained 7,727 assisted units. This report uses information on those units to characterize the 4.5 million rental units assisted by HUD in 2021, and similar information from previous address-matching efforts to analyze how the HUD-assisted rental stock has changed over the last three decades. From 1991 to 2021, the number of HUD-assisted units increased by 11.3 percent, or approximately 455,000 units. HUD’s program mix has changed substantially as the Housing Choice Voucher program expanded its share from 26 percent of the assisted stock in 1989 to 49.4 percent in 2021. Overall, the HUD-assisted stock is in good condition, with only 3.8 percent of units severely inadequate compared with 3.4 percent of unassisted units occupied by very low- income renters.